Infrared thermocouple improvements

ABSTRACT

The output stability of an infrared thermocouple is improved by filtering the radiation received by the infrared thermocouple to pass only short wavelengths. The stability is further increased by providing a second infrared thermocouple having its input filtered to pass long wavelengths. The two outputs are combined to obtain an output signal which is substantially independent of emissivity. The linear range of an infrared detector through which its output closely follows that of a linear thermocouple is increased by a calibration method in which an initial offset is provided to a readout device. Calibration of the infrared detector is completed using an adjustable potentiometer. By providing removable apertures, the temperature range through which an infrared thermocouple may be used is extended. Elongated targets are efficiently viewed by an infrared thermocouple having an elongated thermopile flake and an imaging lens.

BACKGROUND

A widely used temperature detector in industrial and many otherapplications is the thermocouple. A thermocouple comprises two wires ofdissimilar metals joined at hot and cold junctions. A voltage is createdas a function of temperature difference between the junctions. In atypical application, the thermocouple has a first junction of dissimilarelectrical conductors in thermal contact with a material, thetemperature of which is to be detected. The dissimilar conductors passthrough a lead to a temperature controller where they connectelectrically at a second thermocouple junction. The temperature of thesecond junction is maintained at a stable reference level or ismonitored. The voltage generated due to the difference in temperature atthe two junctions then serves as an indication of the temperaturedifference between the temperature being detected and the referencetemperature.

A disadvantage of thermocouples is that they require contact to thematerial being sensed. Radiation detectors such as thermopiles andpyroelectric devices have been used as a noncontact alternative tothermocouples. Radiation detectors are based on the principle that thethermal radiation emitted from a subject is proportional to thetemperature of the subject raised to the fourth power. The radiationemitted is also a function of the emissivity of the subject and ofbackground radiation, but those factors can be calibrated out forapplications in which the target has consistent properties.

A preferred type of radiation sensor is the thermopile. A thermopile hashot and cold junctions, the hot junction being exposed to thermalradiation from a target. The radiation received from the target raisesthe temperature of the hot junction and the temperature differencebetween the hot and cold junctions of the thermopile causes a voltage tobe produced. Thermopiles respond directly to heat flux rather than totemperature, so temperature detection has required close monitoring ofthe temperature of the thermopile cold junction and appropriateprocessing electronics. Whereas thermocouple technology relies oninexpensive thermocouple junctions which may be plugged into widely usedthermocouple controllers, radiation detectors have typically requiredfull electronics associated with each thermopile detector.

The infrared thermocouple presented in U.S. Pat. Nos. 5,229,612 and5,319,202 obtains the advantages of noncontact temperature sensing ofthe radiation detector using a detector element having electronicsimplicity approaching that of thermocouples. Infrared thermocouples maybe coupled directly into thermocouple controllers which are prevalent inthe industrial environment.

In its simplest form, the infrared thermocouple comprises a thermopile,for viewing a target, coupled in series with thermocouple junctions. Onethermocouple junction is located by the thermopile while the otherjunction may be in the thermocouple controller to provide the reference.With such a connection, the thermocouple controller sees the combinedvoltage generated by the thermocouple junctions and the thermopile as itwould see the voltage generated by thermocouple junctions alone. Theremote thermocouple junction eliminates the need for the usual absolutetemperature sensor at the thermopile cold junction. By proper design ofthe thermopile and thermocouple, the combination can be caused to mimicthe output of a conventional thermocouple over a temperature range ofinterest.

Since the thermocouple is a linear device and the thermopile is anonlinear device which has a voltage output proportional to the fourthpower of the target temperature, the infrared thermocouple can only becaused to mimic the thermocouple over a limited temperature range. Toenable calibration of the infrared thermocouple to a desired targettemperature, it is preferred that a potentiometer be connected acrossthe thermopile output to calibrate that output. A typical infraredthermocouple provides an output of plus or minus 2 percent of the linearthermocouple value over a range of about 100° F. (60° C.). With acalibrating potentiometer, the target temperature about which that 100°F. range of linearity occurs can be set within a much wider range oftemperatures.

SUMMARY OF THE INVENTION

The present invention relates to several methods for extending theapplications of the infrared thermocouple. For example, one limitationon prior infrared thermocouples has been the sensitivity to changes inemissivity of the target, particularly for targets of low emissivity.Further, there is a preference to extend the linear range over which aninfrared thermocouple output approximates the linear output of athermocouple.

In accordance with one aspect of the invention, an infrared detectorcoupled to a readout device is adapted to be less sensitive to changesin emissivity with low emissivity targets. The infrared detector ispreferably an infrared thermocouple or other device having a sensorwhich generates voltage in response to infrared radiation in a passivecircuit. Radiation to the infrared thermocouple is filtered to blocksubstantially all radiation of wavelengths greater than 5 microns. Aparticularly beneficial filter is a sapphire filter which has a passband of about 0.1 to 5 microns. By limiting the detected radiation toshorter wavelengths, the sensitivity of the thermopile signal totemperature changes is increased without increasing the sensitivity ofthe signal to emissivity changes.

A disadvantage of the detection of only shorter wavelengths and theresultant increased sensitivity to temperature is that the thermopileresponse is even more nonlinear than with typical infraredthermocouples. Accordingly, the linear temperature range issubstantially reduced. In order to increase the linear range of lowexissivity infrared thermocouples as well as more conventional infraredthermocouples and other infrared detectors, a new calibration method isprovided. Accordingly, an offset to the readout device, such as athermocouple controller, is provided to cause an offset readout evenwhen no radiation input is provided to the infrared detector. Theinfrared detector is then calibrated, using an available calibrationpotentiometer or variable aperture or combination of the two, as thedetector views a target at a known calibration temperature. Withcalibration of the infrared detector, the readout is caused tocorrespond to the readout which would be expected at the calibrationtemperature.

By this calibration technique, the voltage signal seen at the readoutdevice is first raised by the offset through the full temperature rangeand then lowered by means of the potentiometer calibration through onlythe higher temperature ranges. The resultant effect is a rotation of thenonlinear voltage response seen at the readout device. That rotationcauses the voltage seen by the readout device to more closely follow thelinear output of a thermocouple over a substantially larger temperaturerange. In fact, the temperature range of a low emissivity model infraredthermocouple may be increased from about 50° F. (10° C.) to about 300°F. (150° C.).

Prior high emissivity detectors have included 6.5 to 14 micron passfilters for low temperature applications of less than 1000° F. (540° C.)and 2 to 20 micron pass filters for applications ranging from roomtemperature to 5000° F. (2750° C.). The high bandwidth and hightemperature response of the 2 to 20 micron detector is at the cost ofreduced linearity. The linear temperature range of about 100° F. (60°C.) found with the 6.5 to 14 micron infrared thermocouples and otherradiation detectors can be extended to more than 500° F. (280° C.). Thelinear temperature range of the 2 to 20 micron detector can be extendedfrom 50° F. (10° C.) to about 300° F. (150°).

The optimum offset to be added at the readout device is roughly apercentage of the target temperature. For low emissivity models with a0.1 to 5 micron filter, an offset of about 75% of target temperaturereadout is preferred. For the high emissivity models with 2-20 micronspectral range, the optimum offset is about 60%. For very high targettemperatures, an offset of only about 50% is preferred. In general, anoffset of from about 40% to 100% of target temperature results insignificant improvement in linearity.

The sensitivity to changes in emissivity can be further reduced by usingtwo infrared thermocouple detectors, each having a different filter. Ashort wavelength pass filter, such as the sapphire filter, is used forfiltering radiation to one infrared thermocouple and a long wavelengthfilter, such as one which passes wavelengths of 6.5 microns to 14microns filters radiation to the second infrared thermocouple. Theradiation sensed from the target through the two filters and infraredthermocouples provides first and second emissivity dependent outputs.However, those outputs can be combined to provide a temperatureindication which is substantially less dependent on emissivity. Sincethe lower wavelength output is much more dependent on temperaturechanges and both outputs are similarly dependent on emissivity changes,the two outputs together permit the determination of an emissivityindependent temperature readout. At very high temperatures, the longerwavelength output becomes nearly independent of change in temperature.In that case, the two infrared thermocouple outputs can bedifferentially coupled to provide a direct reading of emissivityindependent temperature. A calibration method for that differentiallycoupled infrared thermocouple includes first calibrating the shortwavelength infrared thermocouple using the offset and potentiometeradjustment. Then, the two infrared thermocouples are connecteddifferentially and the potentiometer of the long wavelength infraredthermocouple is adjusted to bring the readout to the initial offsetvalue. Finally, the offset is changed to bring the output to the knowncalibration temperature.

The range of temperatures over which an infrared thermocouple may becalibrated is generally limited by the amount of radiation to which thethermopile may be exposed. Viewing a target of extremely hightemperature may overload the thermopile. Infrared thermocouples adaptedto view extremely high temperatures must have smaller apertures to limitthe amount of radiation which passes to the thermopile. However, suchinfrared thermocouples are then unsuitable for lower temperatureapplications. In accordance with another aspect of the presentinvention, an infrared thermocouple may be easily and inexpensivelyadapted to a very wide range of target temperatures by removablycoupling an aperture to the infrared thermocouple to limit radiationsensed by the thermopile. In one simple implementation, a groove isprovided within an open window through which the thermopile views thetarget. The aperture may be retained within the window by a clip seatedin the groove. A kit of different aperture sizes may be provided and anappropriate aperture may be selected, with smaller apertures being usedfor higher temperature applications.

The fields of view of infrared thermocouples must be designed to meetvarious applications. For large target areas, a wide field of view maybe used to increase the amount of radiation received by the thermopileand thus the sensitivity of the infrared thermocouple. On the otherhand, with small target areas, a small field of view has been used inorder to avoid the detection of background surface temperatures. Inaccordance with yet another aspect of this invention, an infraredthermocouple is adapted to provide maximum sensitivity without sensingbackground temperatures for extended targets such as rods or filaments.Accordingly, the infrared thermocouple has a thermopile sensor regionwhich is elongated along a sensor axis and which is positioned toparallel the target axis. A lens is provided to image the target on tothe sensor region. Although the sensor region may be defined by anaperture which is also imaged onto the thermopile, it is preferred thatthe thermopile flake itself have an elongated shape.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 illustrates an infrared thermocouple with thermocouple controllerembodying the present invention.

FIG. 2 is a cross-sectional view of the infrared thermocouple detector.

FIG. 3 is an electrical schematic of the system of FIGS. 1 and 2.

FIG. 4A illustrates the output signal characteristics with calibrationof an infrared thermocouple to four target temperatures, and FIG. 4B isan enlarged illustration of the output signal characteristics at onecalibrated target temperature.

FIG. 5A illustrates the detected temperature with emissivity errors, andFIG. 5B illustrates the temperature output with those errors minimizedusing principles of the invention.

FIG. 6 illustrates the Planck function at various target temperatures.

FIG. 7 illustrates the improved performance of an infrared thermocouplemodified to minimize emissivity errors in accordance with the invention.

FIG. 8A illustrates the output signal characteristics of an infraredthermocouple modified to reduce emissivity errors, and FIG. 8B is anenlarged illustration of the output signal at one calibrated targettemperature.

FIGS. 9A, B and C illustrate a calibration method which improves thelinear range of an infrared thermocouple.

FIG. 10 illustrates the response of output signal to target temperatureand emissivity with long wavelength pass filtering.

FIG. 11A schematically illustrates differential coupling of two infraredthermocouples.

FIG. 11B illustrates a physical implementation of the differentiallycoupled infrared thermocouples.

Figure 11C illustrates calibration of the differentially coupledinfrared thermocouples.

FIG. 12A is an exploded view of a removable aperture assembly to becoupled to an infrared thermocouple.

FIG. 12B is a cross-sectional view of the infrared thermocouple withinstalled aperture.

FIG. 12C illustrates the potential extended ranges obtainable using theaperture of FIGS. 12A and 12B.

FIG. 13A is an illustration of an application of an elongated flakeembodiment of the invention.

FIG. 13B illustrates alignment lines on the rear surface of the infraredthermocouple of FIG. 13A.

FIG. 13C is a cross-sectional view of the infrared thermocouple of FIG.13A with focusing lens.

FIG. 13D illustrates an image of thermopile flake having an elongatedshape in the embodiment of FIG. 13A.

FIG. 14 illustrates yet another application of the embodiment of FIGS.13A-D.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a typical application of an infrared thermocouple. Aninfrared thermocouple 20 is positioned to view a target surface 22which, in this case, is a sheet of material which moves from right toleft. The dissimilar thermocouple wires extend through a lead 24 to aremote readout device 26. The readout device 26 may be any conventionaldevice adapted to receive thermocouple leads and provide a display ortransmit an output such as thermocouple controllers, PLCs, meters andtransmitters.

A cross-sectional schematic of the detector 20 is shown in FIG. 2. Athermopile flake 28 is positioned within a sealed can 30 having a rigidinfrared transparent window 31. The conventional thermopile cansurrounds the thermopile flake in a xenon environment. In order tonarrow the field of view of the infrared thermocouple detector, a lens32 may be provided. Preferably, the lens is a Fresnel lens.

FIG. 3 illustrates a preferred circuit of the infrared thermocoupledetector. A thermopile 34 includes the thermopile flake 28 and producesa voltage output in response to the temperature differential createdbetween the hot and cold junctions of the thermopile 34. In thiscircuit, the thermopile 34 is shown as its electrical equivalents of avoltage source V_(TP) and in internal resistance R_(TP). A useradjustable potentiometer 36 is coupled directly across the thermopile 34to allow for calibration of the thermopile output using an externaladjustment screw 38 (FIG. 2). A temperature dependent resistor 38, incooperation with parallel resistor 42 and series resistor 40, make theoutput taken across resistor 40 generally independent of cold junctiontemperature.

The thermopile circuit is connected in series with the thermocouplejunction 44 between dissimilar conductors 46 and 48. Those conductorsmay be of any known thermocouple material. Conductor 50 is of the samematerial as conductor 48. Since the connections 52 and 54 are maintainedat a common temperature, the thermopile circuit does not effect thethermocouple function, but only adds the voltage output of thethermopile circuit to that of the thermocouple. The reference junctionof the thermocouple is formed at 56 in the readout device 26. Thevoltage across the resistor 58 is then the sum of the thermopile outputvoltage across resistor 40 and the thermocouple voltage resulting fromthe temperature difference between remote junction 44 and referencejunction 56. In conventional thermocouple readout devices, the junction56 is maintained at a stable reference temperature or its temperature ismonitored.

FIGS. 4A and B illustrate the output signal from a typical infraredthermocouple as compared to that of a type S thermocouple. Thethermocouple output signal 60 changes linearly with change in targettemperature. Each of the nonlinear curves illustrates the output of aninfrared thermocouple at a different calibration setting of thepotentiometer 36. In each case, the output is a power function of targettemperature. By adjusting the potentiometer 36, the output can be raisedor lowered to cause the output signal to match the output signal of athermocouple at a lesser or higher temperature. For example, if in aparticular application the infrared thermocouple was to monitor a targettemperature at about 4000° F., the potentiometer 36 would be adjusted tobring the output signal to the curve 62 which crosses the thermocoupleline at 64 and about 4000° F. The infrared thermocouple would thenprovide an output signal which approximates the linear output of thethermocouple over a limited range of temperature of about 100° F.

FIG. 4B is an enlarged view of the portion 66 of FIG. 4A. It can be seenthat at the calibration temperature of about 2300° F. the output of theinfrared thermocouple directly matches that of a thermocouple at thesame temperature. However, at lesser temperatures, the output is lowerand at higher temperatures the output is above that of the thermocouple.However, within a linear range of about 100° F., the infraredthermocouple output does not vary from that of a thermocouple by morethan ±2%. In some applications, a greater disparity from theconventional thermocouple output can be tolerated for a wider linearrange.

One aspect of the invention provides greater output stability withvariations in emissivity. Emissivity is the property of a material'ssurface that describes its "efficiency" at emitting thermal radiation.An emissivity value of 1.0 represents emission at 100%, and 0 describesemission at 0%.

For non-metals and coated metals emissivity is very high, 0.8 andgreater, and variations are usually not a problem. For example, for aproduction process in which a non-metallic material of emissivity of 0.9is to be controlled, and normal material variations cause emissivityvariations of ±0.01, the associated temperature error will be of theorder of 0.01 divided by 0.9, or -1% of reading, an acceptablevariation. In contrast, if we are to control the temperature of a metalwith emissivity 0.2, then variations of ±0.01 will produce an error ofthe order of (0.01/0.2) or ˜5% of reading. Additionally, metal finishes,which play a significant role in emissivity, tend to cause morevariations than changes in finish in non-metals.

FIG. 5A illustrates a temperature reading of the constant temperaturetarget 22 of FIG. 1 as the target is scanned. The lack of readoutstability results from the differences in emissivity illustrated inFIG. 1. With the present invention the temperature readout can bestabilized as illustrated in FIG. 5B.

A filter design of the present invention filters out the effects ofthese emissivity variations on measured temperature by approximately afactor of four and thus reduces the errors by a factor of four. Thus,with the filter, the errors are of the same order as those commonlyexperienced for high emissivity targets.

The method takes advantage of the basic physics of thermal radiation, inwhich the mathematical description of the energy distribution is by aformula called the Planck function: ##EQU1## where q.sub.λ is radiatedenergy at a given wavelength, ε is the emissivity, T the absolute targettemperature, λ the wavelength, and the other symbols are for variousphysical constants. The Planck function for three temperatures isillustrated in FIG. 6. The Planck function integrates to the morefamiliar Stefan-Boltzman equation: ##EQU2## when all wavelengths aremeasured.

The low emissivity filter works by measuring the energy content of theradiation, as described by the Planck function, over wavelengths thatare more selectively sensitive for temperature variations, and thereforeproportionately less sensitive to emissivity variations, as follows:##EQU3##

If we compute the partial derivative of each expression with respect toemissivity and temperature, we obtain the following relations for theslope of the signal with respect to temperature divided by the slope ofthe signal with respect to emissivity:

Accordingly, by optimum selection of the wavelengths to be measured, thesensitivity to emissivity variations can be significantly reduced, i.e,filtered, by enhancing the relative sensitivity to temperature. Inpractice, the best wavelengths are the shorter ones, since they providethe most sensitivity to temperature, and the least sensitivity toemissivity, as is predicted by the integration of the Planck function.##EQU4##

The "filtering factor" for the low emissivity model is based on theselection of 0.1 to 5 micron for the measured wavelengths and results ina factor of from four to six error reduction, depending on targettemperature, as illustrated in FIG. 7. In a preferred implementation,the filtering is obtained by a sapphire filter 68 (FIG. 2).

As an additional benefit of the low emissivity filter, errors due tosuch factors as smoke, dust, moisture, etc. which may partially blockthe optical path to the target, are also filtered. These factors behavemathematically identically to emissivity, and therefore will be filteredby the same factor of four to six.

An unfortunate disadvantage of the filtering to low wavelengths is anincreased nonlinearity of the output signal as illustrated in FIGS. 8Aand 8B compared to FIGS. 4A and 4B. In FIGS. 4A and 4B, the outputsignal varies to the fourth power of target temperature; whereas, inFIGS. 8A and 8B the output signal varies to the power x which is greaterthan 4. As a consequence, the output signal at a calibration temperatureas illustrated in FIG. 8B changes much more steeply and thus deviatesfrom the linear thermocouple temperature output much more significantly.Thus, to maintain a difference from the linear output of less than 2%,for example, a much lesser range of temperature variation is permitted.

A calibration approach which extends the linear range of the filteredoutput and which may be used even with the nonfiltered outputs, isillustrated in FIGS. 9A, B and C. A typical narrow linear range forwhich the output approximates the linear thermocouple output isillustrated in FIG. 9A. This range may be extended by the followingcalibration technique. Before calibrating the infrared thermocouple withthe potentiometer 36, the readout device is provided with an offsetwhich raises the output signal as illustrated in FIG. 9B. As previouslyillustrated in FIGS. 4A and 8A, calibration with the potentiometer 36results in a greater change in high temperature response which can beseen as a rotation of the output signal curve. Accordingly, asillustrated in FIG. 9C, calibration of the offset output signal rotatesthe signal curve to cause it to meet the linear thermocouple output at adesired calibration point. This rotation of the output signal has causedthe signal to follow the thermocouple linear output over a substantiallywider linear range as illustrated in FIG. 9C. This offset can beobtained using the standard OFFSET, ZERO LO CAL, or equivalentadjustment 88 (FIG. 1) of a conventional readout device.

The optimum offset for extending the linear range has been found to be apercentage of target temperature. For example, for the output of thefiltered infrared thermocouple of FIG. 8A, an offset corresponding to75% of the target temperature is best. For the output of a nonfiltereddevice (FIG. 4A), an offset of about 60% of target temperature isgenerally preferred, but at high temperatures of about 3000° F. (1650°C.) an offset of only 50% of target temperature is preferred. Ingeneral, an offset of about 40% to 100% provides significantimprovements in linear range. For example, a 50° F. linear range can beextended to 300° F.

Consider the example of monitoring steel at 1800° F. (980° C.). Coverthe infrared thermocouple with aluminum foil such that it cannot see thetarget. Then set the readout device offset so that the display readsapproximately 75% of target temperature; 0.75×1800=1350° F.(0.75×980=735° C.). Remove the foil, point the infrared thermocouple atthe intended target, and adjust the calibration screw 38 (FIG. 2) on theback of the infrared thermocouple until the readout display reads thecorrect temperature. The calibration is complete, and the linear rangeover which the reading will be within 2% of actual is approximately1800° F.±250° F. (1350° C.±140° C.).

Though filtering the radiation to pass only short wavelengths increasesthe stability with changes in emissivity, even greater stability can beobtained by taking two measurements, one at short wavelengths andanother at longer wavelengths. Prior infrared thermocouples have usedsilicon optics to pass wavelengths of 7 microns to 20 microns. Suchfilters have been used to linearized the output by removing the verynonlinear response found at the shorter wavelengths. FIG. 10 illustratesthe response of the signal output in the long wavelength range and canbe compared to the short wavelength output of FIG. 8A. It can be seenthat, at lower temperatures, the output is a near linear function oftemperature, while at higher temperatures, the output levels off to anear constant value dependent principally on change in emissivity.Accordingly, the long wavelength signal can be seen to be a linearfunction of temperature or

    s.sub.lw =c.sub.1 εf.sub.1 (T)

Thus the long wavelength signals can be seen to be strongly dependent onemissivity. The short wavelength signal, on the other hand, is stronglydependent on temperature:

    s.sub.sw =c.sub.2 εf.sub.2 (T.sup.x)

Alternatively, the second measurement could be of wide band withoutfiltering and thus be a power of four. By taking the two measurements,the emissivity can be removed as a variable in determining temperature.

At very high temperatures, where the long wavelength signal is much lessdependent on temperature than it is on emissivity, a differentialmeasurement of the two signals can be taken. A differential reading canbe taken from the circuit of FIG. 11A. This circuit can be seen to beidentical to that of FIG. 4 of U.S. Pat. No. 5,319,202 except that thewindows 90 and 92 are filters of short and long wavelength bandsrespectively. Specifically, a short wavelength band of 0.1 to 5 micronsand a long wavelength band of 6.5 to 14 microns are preferred. Thus,there is provided a first detector 100 having a thermopile 101 and acalibration potentiometer 94 coupled in series with a thermocouple 102having junctions 96 and 106, 107. The second infrared thermocoupledetector 97 comprises a thermopile 103 having a calibrationpotentiometer 98. A thermocouple 104 comprises a junction 99 and ajunction 108, 109. When coupled together in series, the combined outputs_(d) is the difference between s_(sw) and s_(lw). Since both s_(sw) ands_(lw) vary linearly with emissivity, any changes in s_(d) would beindependent of change in emissivity. Changes in the output would thus bedependent on temperature only. The differential readout is insensitivenot only to emissivity changes in the target, but to any condition whichaffects both detector outputs equally. For example, smoke and commonobstructions will not change the differential output.

Although the use of a thermocouple in series with the thermopile isgenerally preferred it is not always required. In particular, thethermocouple output becomes less significant as target temperatureincreases.

A physical implementation of the differential measurement is illustratedin FIG. 11B. The short wavelength detector 100 and long wavelengthdetector 97 are mounted such that they both view the same target area.They are connected in series opposition to the thermocouple inputs of amonitor or controller 95. The controller drives a heater at the target91.

One method of calibrating the differential system is illustrated in FIG.11B. First calibrate the short wave infrared thermocouple to the targettemperature with the long wave infrared thermocouple covered with foil.Accordingly, if the target temperature were 2000° F., the offset of thereadout device would be set to 75% of that temperature or 1500° F. Then,while viewing the target at that temperature, the short wavepotentiometer 94 is adjusted to bring the output to 2000° F. The shortwave response is thus rotated form S_(sw1) to S_(sw2). Then the longwave infrared thermocouple is exposed to the target. The potentiometer98 would be adjusted to bring the output reading to 1500° F., the meterreading with a zero signal, thus assuring that s_(sw2) =s_(1w). Finally,the offset of the readout device is adjusted to bring the readout to thetarget temperature of 2000° F.

The above calibration method is particularly suited to systems whichrequire an accurate temperature indication over some range oftemperatures. In a controller implementation where the sensed signal isused to control the temperature and thus maintain a particular operatingtemperature, a simpler calibration technique may be used. With thetarget surface heated to the operating temperature, the outputs of theindividual detectors are determined using a digital voltmeter. By meansof the scaling potentiometer 94 or 98, the detector having the highestoutput is adjusted to cause that detector output to equal the output ofthe other. The two detectors are then connected in series oppositionsuch that the combined signal to the controller 95 is zero. Thedetectors are wired such that the short wavelength unit increasestemperature displayed with an increase in signal and the high wavelengthunit decreases temperature display with increase in signal. Thecontroller offset is then adjusted so that the differential signalcauses a reading of the actual target temperature on the controller. Inthis calibration technique, no attempt is made to flatten the responseof either detector since, with any drift in either direction, thecontroller brings the temperature back to the predefined operatingtemperature to maintain a constant reading at the operating temperature.

Although a variable resistor has been illustrated as the means forcalibrating the detectors, it will be recognized that a variableaperture can serve the same function. The variable resistor is preferredsince a tuned aperture is more difficult and costly to implement.Further, in some applications a detector can be calibrated byappropriate selection of a fixed resistor.

FIGS. 12A, B and C illustrate the use of an aperture kit for roughdetector calibration to extend the available target temperature rangefor a given infrared thermocouple, improve the adjustment sensitivity ofthe adjustment potentiometer and to reduce the minimum spot size. In apreferred example, an aperture kit comprises two apertures of one halfinch (13 mm) and one quarter inch (6 mm) of stainless steel and tworetaining rings. As illustrated in FIG. 12A, the internal surface of thehousing 110 of the infrared thermocouple is threaded. The threadprovides a groove in which retaining clips 112 and 114 may be seated toretain an aperture 116 therebetween as illustrated in FIG. 12B. Theaperture reduces the quantity of radiated energy entering the infraredthermocouple optical system, thus increasing the rated maximum targettemperature before burnout. In addition, since less signal is producedat a given temperature, the adjustment will be less "tweaky" whencalibrating the installation.

An example illustration of how the apertures can extend the range ofseveral infrared thermocouple models is presented in FIG. 12C. Thevalues 100, 20, and 10 indicate the normal field of view of thedetector, that is the ratio of distance from the detector to spot size.Those fields of view are determined by the lens optics of the detectors.The cross-hatched regions indicate the extended temperatures over whicheach of the detectors may be viewed.

Measurement of very thin targets, such as extrusions as illustrated inFIG. 13A, can be very difficult. In order to avoid detection ofbackground surface area, the field of view of the infrared thermocouple120 should be no greater than the width of the extrusion 122. To avoidthe extreme loss in sensitivity with such a small field of view, theinfrared thermocouple 120 includes two modifications: a focusing lenswhich enables the user to image the thermopile flake onto the narrowextrusion and a thermopile flake which is itself elongated so that itcan be aligned with the extrusion to sense an extended length of theextrusion without sensing beyond the width of the extrusion.Accordingly, FIG. 13D illustrates an image of the thermopile flake 124.As illustrated in FIG. 13C, the focusing lens 126 enables the filament122 to be focused directly onto the flake. As an alternative to actuallyshaping the thermopile flake to control the sensing region, additionaloptics may be included to image both the flake and the target onto anelongated aperture.

As illustrated in FIG. 13B, the rear surface of the infraredthermocouple is scribed with lines 123 which assist in alignment of theflake parallel to the target. Alignment of the infrared thermocouplewith the extrusion 122 can be fine tuned by moving the sensor closer,farther and rotated slightly until a maximum signal is obtained.

An alternative use of the infrared thermocouple 120 having a focusinglens and elongated flake is illustrated in FIG. 14. In this case, alarge target is positioned behind a narrow slot. To view the target, theflake is imaged onto the opening to view a larger field 130 of thetarget 132.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method of detecting temperature of a target byan infrared detector with reduced emissivity sensitivitycomprising:providing first and second infrared detectors, a first passband filter for passing a first wavelength range of radiation to thefirst infrared detector and a second pass band filter for passing asecond wavelength radiation to the second infrared detector the secondrange including longer wavelengths than the first range; sensingradiation from a target through the first filter and first radiationdetector to provide a first emissivity dependent output; sensingradiation from the target through the second filter and the secondinfrared detector to provide a second emissivity dependent output; andelectrically coupling the first and second emissivity dependent outputsto subtract one from the other and provide a temperature signal which issubstantially less dependent on emissivity than are the first and secondemissivity dependent outputs.
 2. A method as claimed in claim 1 whereineach of the first and second infrared detectors is an infraredthermocouple.
 3. A method as claimed in claim 1 further comprisingcalibrating the first and second infrared detectors by:coupling thetemperature signal to a readout device: providing an offset to thereadout device to cause an offset readout with no radiation input to theinfrared detectors; causing the first infrared detector to view a targetat a calibration temperature; calibrating the first infrared detector tocause the readout to correspond to the calibration temperature; causingthe second infrared detector to view the target at the calibrationtemperature; calibrating the second infrared detector to cause thereadout to correspond to the offset readout; and changing the offset tothe readout device to cause the readout to correspond to the calibrationtemperature.
 4. A method as claimed in claim 1 further comprisingcalibrating the first and second infrared detectors by:coupling thetemperature signal to a readout device: calibrating at least one of theinfrared detectors to provide equal outputs from the detectors whileviewing a common target at a calibration temperature; and changing anoffset to the readout device to cause a readout of the temperaturesignal to correspond to the calibration temperature of the target.
 5. Amethod as claimed in claim 1 wherein the first passband filter blockswavelengths greater than about 5 microns.
 6. A method as claimed inclaim 5 wherein the second passband filter has a passband of about 6.5microns to 14 microns.
 7. A method as claimed in claim 1 wherein eachinfrared detector comprises a sensor which generates voltage in responseto infrared radiation in a passive circuit.
 8. A method of detectingtemperature of a target by an infrared detector with reduced emissivitysensitivity comprising:sensing radiation from a target in a firstwavelength passband by a first radiation detector to provide a firstemissivity dependent output; sensing radiation from the target in asecond wavelength passband by a second infrared detector to provide asecond emissivity dependent output, the second wavelength passband beingdifferent than the first wavelength passband; and electrically couplingthe first and second emissivity dependent outputs to subtract one fromthe other and provide a temperature signal which is substantially lessdependent on emissivity than are the first and second emissivitydependent outputs.
 9. A method as claimed in claim 8 wherein eachinfrared detector comprises a sensor which generates voltage in responseto infrared radiation in a passive circuit.
 10. A method as claimed inclaim 8 wherein each of the first and second infrared detectors is aninfrared thermocouple.
 11. A method as claimed in claim 8 furthercomprising calibrating the first and second infrared detectorsby:coupling the temperature signal to a readout device; providing anoffset to the readout device to cause an offset readout with noradiation input to the infrared detectors; causing the first infrareddetector to view a target at a calibration temperature; calibrating thefirst infrared detector to cause the readout to correspond to thecalibration temperature; causing the second infrared detector to viewthe target at the calibration temperature; calibrating the secondinfrared detector to cause the readout to correspond to the offsetreadout; and changing the offset to the readout device to cause thereadout to correspond to the calibration temperature.
 12. A method asclaimed in claim 8 further comprising calibrating the first and secondinfrared detectors by:coupling the temperature signal to a readoutdevice; calibrating at least one of the infrared detectors to provideequal outputs from the detectors while viewing a common target at acalibration temperature; and changing an offset to the readout device tocause a readout of the temperature signal to correspond to thecalibration temperature of the target.